Class D amplifiers, also called pulse-width-modulation (PWM) amplifiers, have been known in the art for more than half a century. In a typical Class D amplifier, the output signal switches on and off at a variable rate according to an input signal. Typical Class D amplifiers employ only switching processes in their output system and can theoretically be highly efficient. However, use of Class D amplifiers for high fidelity audio is relatively rare.
Class D amplifier output circuits typically employ either a half-bridge circuit, as shown in FIG. 1, or a full bridge (H-bridge) circuit, as shown in FIG. 2 to recover a continuous signal. The use of an H-bridge allows operation with a single-polarity power supply to generate two mirror-image output signals, which are bridged to supply the output. Both half-bridge and full-bridge amplifiers produce the classical two-state operation (+/-V) of Class D amplifiers. These bridged amplifiers are sometimes now referred to as Class AD amplifiers.
There are numerous disadvantages to Class AD operation. The most serious problems result from the two-state operation format, which can distort the output signal during demodulation. A high level of output ripple voltage exists at low signal levels. It is thus difficult to design LC low-pass filters of suitable carrier rejection without the AC impedance of the low-pass filter causing serious attendant power limitations at upper audio frequencies. This two-state operation also yields low efficiency at low signal levels because of strong circulating currents in the resonant LC circuit of the amplifier. These circulating currents cause heating in the resistance of the inductor and other amplifier components. High output ripple voltages also limit the effectiveness of negative feedback.
Three-state operation Class BD systems were devised to overcome some of these deficiencies of Class D and Class AD operation. As disclosed by U.S. Pat. No. 3,629,616 to Walker, titled High Efficiency Modulation Circuit for Switchmode Audio Amplifier, a Class BD amplifier generates sets of variable-width pulse trains of either positive or negative polarity at an instant signal condition. The generated signal returns to zero volts between pulses. The three-state operation is known as Class BD operation because pulses of only one polarity actually "carry" the signal at a given signal polarity, and the output is similar to the operation of a traditional Class B power amplifier.
In a typical Class BD system, as shown in FIG. 3, two appropriate Pulse Width Modulated (PWM) switching waveforms are formed by dual sampling a triangular carrier waveform.
An example of sampling a triangular waveform by an audio input signal V.sub.in, as shown in FIG. 4. In-phase carrier waves and opposite-phase audio waves are often used to generate the PWM switching waveforms. The two PWM switching waveforms are then amplified to appear at two conventional output points 110, 112 of an H-bridge, as shown in FIG. 3. Each output waveform passes through a respective filter inductor 114, 116 and into a single common filtering capacitor 118. A load 120 parallels the filtering capacitor. The output of this system floats and is the difference between the dual-sampling PWM waveforms. This dual sampling process results in final PWM output switching pulses at twice the carrier frequency (2F) and having a, single polarity which follows the polarity of the instant input signal.
Class BD systems have significant advantages at low output levels, where the final output pulse widths approach zero, unlike Class AD systems, which approach 50%. At these low output levels, the output ripple voltages and circulating current losses in the LC circuitry also approach zero, which allows for very high efficiency at all signal levels. Furthermore, such Class BD systems have advantages over Class AD systems. For example, the second order LC filter of the Class BD system attenuates the twice-frequency ripple in the transformed wave by 12 dB more than in a conventional Class AD system. In addition, the output pulses have half the amplitude of a comparable two-state, Class AD system because they are only one polarity at a time. This half amplitude provides an additional 6 dB attenuation. Thus, Class BD systems yield a total advantage of 18 dB over a comparable Class AD system.
Audio power amplifiers of any class which must output their signal on two non-grounded terminals are generally considered less than fully desirable and prohibit applications requiring that one output terminal be at ground potential. The situation is far more troublesome in PWM amplifiers. An excellent analysis of differential-output Class BD systems was presented by J. Vanderkooy, in Preprint 3886 for the 97.sup.th Convention of the Audio Engineering Society, November 1994, entitled "New Concepts in Pulse-Width Modulation." In that analysis, Vanderkooy cautions that H-bridge operation of Class BD designs results in a virtually unsolvable design conflict in the low-pass LC demodulation filter. Although the arrangement shown in FIG. 3 results in full carrier cancellation when viewed differentially across the H-bridge output terminals, the configuration leaves full-level, common-mode Class AD output signals present on both terminals with respect to ground, which presents a serious and difficult EMI problem.
Filters using capacitors to ground are successful for conventional Class AD amplifiers. Such filtering changes the Class BD operation back into two, independent Class AD outputs, introduces the associated disadvantages of circulating current losses, and virtually negates the dominant motivation of adopting Class BD circuits over Class AD circuits.
Vanderkooy also notes also that his first evaluation of Class BD systems for the cited 1994 AES Journal paper employed a system which forms the parallel sum of the two output legs of an H-bridge, each carrying a Class AD signal. In the Vanderkooy system the output carriers are out of phase, and the audio modulation in phase, as shown in FIG. 5. The carrier signals cancel as they are summed through inductors in each output leg, whereas the demodulated audio signals add. A similar connection is shown by Stanley in U.S. Pat. No. 5,657,219 and by Gulczynski in U.S. Pat. No. 4,980,649. This summation approach avoids the EMI problem inherent to the device shown in FIG. 3, but leaves the efficiency losses in the output inductors, because each LC filter sees the same carrier signals as in Class AD (i.e., 50% duty cycle pulses at zero input signal).
In U.S. Pat. No. 4,020,361, Suelzle discloses a Switching Mode Power Controller of Large Dynamic Range. Suelzle discloses a differential method for forming very short pulses which avoids the need for opening and closing a single switch in rapid sequence to produce a Class BD pulse train. In U.S. Pat. No. 4,162,455, entitled Amplifier Systems, Birt discloses a method for canceling the switching frequency in the output by modulating two separate Class D amplifiers by symmetrically interlaced clock pulse trains. A description of another Class BD modulation technique which is considered "high efficiency" is in U.S. Pat. No. 5,014,016 for a Switching Amplifier to Anderson.
All of the Class BD amplifiers discussed above can throughput weak signals, as they depend on Suelzle's teaching described above. However, none of the Class BD patents discussed above teach how such subtractive systems are able to function on very weak signals, such as one 1,000 times weaker than a signal having a pulse-width equal to system rise-time.
To reproduce 100 dB of dynamic range, a PWM system must reproduce a demodulated signal equating to pulses of 1/100,000 of the system's typical longest pulses. Thus, pulses of 20-50 picoseconds appear to be required, which are signal pulses of one-thousandth of the typical rise-time of the switches (e.g. 30 nanoseconds). Such short pulses are clearly not feasible and are fortunately not required.
A Class BD system changes operation dramatically for signal pulses narrower than rise-time, as shown in FIG. 6. In Class BD systems, two PWM signal edges 122 and 124 are subtracted to represent a signal of width t.sub.P, less than system rise time t.sub.R. Analysis by similar triangles shows that a new, equivalent pulse of width t.sub.R is formed with reduced amplitude a=A(t.sub.P /t.sub.R). The pulse "value" is thus accurately preserved because the process creates a substitute pulse of area equivalent to the irreproducible pulse by increasing width proportionally to the reduction of amplitude. Thus, in a system having 50 nanosecond rise time, a signal level which equates to a 50 picosecond pulse-width will have an amplitude of only 1/1000 of the normal pulse amplitude, but will be spread out 1000 times wider. Thus, pulse amplitude modulation (PAM) may be used when Pulse Width Modulation (PWM) becomes unfeasible.
Because the filtering process following the subtraction step in subtractive Class BD amplifiers employs only passive, linear components, the passive filter will convert a given pulse area of any height to the appropriate instant output signal. The passive filter is blind to the pulse reshaping activity described above. Thus, subtractive Class BD systems maintain dynamic range for pulse durations vastly less than system rise time and eliminate the need for elaborate and costly digital algorithms to correct pulse-width distortion caused in three-switch systems by switch rise-times (as shown, for example, in U.S. Pat. No. 5,617,058 to Adrian et al.).
In addition, all of the Class D systems discussed above effectively keep the load always switched to a low-impedance source of potential, including ground, thus generating an inherently low output impedance before the LC filter. However, the required LC filter contributes considerable impedance and frequency-response aberrations in the upper range of audio frequencies. This problem has been particularly troublesome in Class AD designs in which the high level of ripple has effectively precluded use of sufficient negative feedback to counteract the filter aberrations.